The present study gives a possible explanation for the ubiquitous occurrence of 6-methyl-5-hepten-2-one and acetone in ambient air and reports for the first time on a widespread occurrence of geranyl acetone and 4-oxopentanal. We have conducted a series of laboratory experiments in which it is demonstrated that significant amounts of geranyl acetone, 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA) and acetone are formed by the reaction of ozone with foliage of common vegetation in the Mediterranean Sea.